The perennial herb soapwort, Saponaria officinalis, owes its prized
cleansing foam to detergent-like compounds called "saponins."
But soapwort isn't the only plant that produces the compounds; nor
are their properties limited to removing dirt and grime.

In studies at the Agricultural Research Service's National
Center for Agricultural Utilization Research (NCAUR) in Peoria,
Illinois, scientists are spiking laboratory diets fed to corn earworm
and fall armyworm with saponins from soybeans, switchgrass, yerba mate,
and other sources to determine what effects the compounds have on the
caterpillar pests' growth and survival.

ARS entomologist Pat Dowd says the studies are an integral part of
a broader effort at Peoria to identify novel sources of resistance that
can be put into corn--either through traditional plant breeding or
biotechnological means. Ultimately, this could usher in new corn
varieties that sustain less damage from caterpillars, are less prone to
infection by toxin-making molds, or require fewer pesticide
applications. Another potential benefit is staving off the ability of
pests like corn earworms to build tolerance to existing sources of
resistance--such as that endowed by insecticidal proteins from the soil
bacterium Bacillus thuringiensis, which is used in about 63 percent of
U.S. corn, according to USDA's Economic Research Service.

[ILLUSTRATION OMITTED]

"Looking for natural methods of controlling pathogens and
pests is a win-win situation for the environment, for businesses who
want to grow their efforts in green technologies, and ultimately for the
U.S. taxpayer, who benefits from a cleaner environment and a thriving
economy," remarks Alejandro Rooney, who leads NCAUR's Crop
Bioprotection Research Unit, where Dowd, ARS molecular biologist Eric T.
Johnson, and others are evaluating the insecticidal properties of
saponins and other natural compounds.

Making Sense of Saponins

In nature, the saponins are produced in the stem, seed, roots,
leaves, or fruit of plants belonging to more than 100 different
families, including Allianceae, Caryophyllaceae, Rosaceae, and Gramineae
(of which corn and switchgrass are both members). But until recently, no
cross-cutting studies have been done comparing the degree to which
saponins from different plant species or families confer resistance to
pests and pathogens, note Dowd, Johnson, and colleagues in an upcoming
issue of the Journal of Chemical Ecology.

"Most grain crops, except for oats, do not have saponins in
them," Dowd says. Why this is so remains a mystery. But studying
close relatives of today's grain crops may reveal important clues.
For example, examining saponins that make some types of switchgrass less
palatable to fall armyworms may reveal dormant genes or biochemical
pathways in a distant relative like corn that can be activated via
genetic engineering or conventional breeding.

"Theoretically, selecting plants for beneficial agronomic
traits could have resulted in the loss of pest-resistance genes,"
says Dowd. "If these resistance genes were located near alleles of
genes that conferred undesirable agronomic traits, they may have been
bred out along with the undesirable genes during the process of
developing commercial lines."

Investigating the Lignin Connection

A crucial first step in studying a compound is to determine its
biological activity, structure, and expression levels. This past year,
Dowd, Johnson, and ARS chemist Mark Berhow conducted experiments in
which they force-fed 1 of 10 different kinds of saponins, caffeine, and
other compounds to first-stage fall armyworms and corn earworms. The
team purchased two of the saponins, a steroidal type called
"diosgenin" and a related form called
"protodioscin," after being alerted to their presence in
switchgrass by Ken Vogel at ARS's Grain, Forage and Bioenergy
Research Unit in Lincoln, Nebraska.

[ILLUSTRATION OMITTED]

Vogel's team is interested in learning whether the saponins
played a part in the resistance of some low-lignin lines of switchgrass
to fall armyworms. The group had previously developed the low-lignin
lines to expedite fermentation of the plants' sugars into ethanol.
One concern was that reducing the lignin content would also make the
plants more vulnerable to chewing by fall armyworms. But in trials, 7 of
the 14 switchgrass lines in fact resisted the caterpillar pests, though
this depended on the timing of the growing season.

"This suggests there is a temporal as well as a genetic
component to expression of the fall armyworm resistance," says
Gautam Sarath, an ARS molecular biologist at Lincoln who is
collaborating with Vogel and ARS agronomist Rob Mitchell to explore
switchgrass's potential as a commercial ethanol crop.

Sarath says it is possible that diosgenin, which has been linked to
digestive problems in livestock, may have compensated for the reduced
lignin by helping wreak similar havoc on fall armyworms that attempted
to feed on the low-lignin lines during tests.

In the lab tests at Peoria, protodioscin had some activity against
fall armyworms, as did saponins from mate, soap bark tree, and soybeans.
One type of soy saponin, called "soyasaponin B," which has a
sugar molecule attached, proved more effective than its sugar-free form
against corn earworm caterpillars, reducing their growth by more than 50
percent.

Dowd notes that they evaluated different sources of saponins
because some insect pests, like fall armyworms and corn earworms, are
generalist feeders, but still have different host preferences. Thus, it
was expected that saponins the pests don't typically encounter in
nature would be more toxic to them. But this wasn't necessarily the
case. Rather, the difference seemed to come down to whether the saponins
harbored certain sugar groups.

None of the saponins tested killed either of the two pest species.
But a smaller caterpillar isn't necessarily a failure: Under field
conditions, plants that taste bad can cause the caterpillars to seek out
other plants; this could translate to reduced kernel damage as well as
easier pickings for hungry predators.

Giving Pests the Blues

Besides saponins, the Peoria researchers are also evaluating the
pest-fighting potential of phytochemicals such as anthocyanins, which
give blueberries, plums, grapes, and flowers like petunias, for example,
their blue and purple colors.

In feeding experiments, corn earworm larvae forced to feed on blue
areas of petunia petals gained less weight than larvae that fed on white
areas. Additional feeding experiments determined that anthocyanins
isolated from the petunia petals also slowed the larvae's growth.
Cabbage looper larvae that fed on blue areas of one petunia
cultivar's petals died at higher rates than larvae that fed on
white areas.

Although it's unclear what petal compound or compounds were
involved in looper mortality, the anthocyanins apparently increased the
effectiveness of the toxic compound.

In other work, says Johnson, "We're also very interested
in proteins in maize that are produced at the seedling stage. The
seedlings are quite resistant to insects, and this may be partly due to
a combination of resistance biochemicals and proteins."

If the proteins' resistance role can be confirmed, then it may
be possible to express the genes responsible for them at a later stage
in the plant's life cycle. "But this would be a matter of
expressing them at sufficient levels," Johnson adds.

Pest-Fighting and People-Friendly

Berhow, who's in NCAUR's Functional Foods Research Unit,
is pursuing another line of inquiry. Some of the same phytochemicals
that plants make to cope with stress caused by insects and pathogens
also benefit people and livestock. For example, studies by Berhow and
University of Illinois (UoI) colleagues indicated that some soybean
saponins have potential as cancer-fighting agents known as
"chemoprotectants." Indeed, in test-tube experiments,
"group B" saponins reduced cancerous human colon cell growth
by 27 to 68 percent.

[ILLUSTRATION OMITTED]

[ILLUSTRATION OMITTED]

In addition to furnishing Dowd with purified material for the
insect trials, Berhow has put his analytical chemistry skills to use
assisting Elvira DeMejia, a UoI associate professor who is studying
anti-inflammatory and other beneficial properties of saponins from yerba
mate leaves, which are used to make a popular South American tea. Like
other saponins tested, mate's saponins also deterred caterpillar
feeding.

Ultimately, such multifaceted studies could converge, giving rise
to new crop varieties that boast dual-use phytochemicals. In furtherance
of that possibility, scientists are continuing research to understand
the full biological activity and effect of saponins in humans and
livestock.--By Jan Suszkiw, ARS.

This research is part of Food Safety (Animal and Plant Products)
(#108) and Quality and Utilization of Agricultural Products (#306), two
ARS national programs described at www.nps.ars.usda.gov.